1. Field of the Invention
One embodiment of the invention disclosed herein relates to contactless power transmitting devices, contactless power receiving devices, and contactless power feeding systems.
2. Description of the Related Art
Various electronic appliances have spread, and a variety of products is shipped to the market. In recent years, portable electronic devices such as cellular phones and digital video cameras have widely spread. Further, electric propulsion machines that are powered by electric power, such as electric cars, appear on the market as products.
In such cellular phones, digital video cameras or electric propulsion machines, power storage units (referred to as batteries and storage batteries) are incorporated. Currently, such a power storage unit is charged in most cases by being directly connected to a household AC power source which is a power feeding means. In a structure which has no power storage unit or which does not use power stored in a power storage unit, a device is operated by power supplied directly from a household AC power source via a wire or the like.
On the other hand, methods by which power storage units are charged without making contact or power is supplied to loads without making contact have been researched and developed. Typical methods are an electromagnetic coupling method (also referred to as electromagnetic induction method, see Reference 1), a radio wave method (also referred to as microwave method), and a magnetic resonance method (also referred to as resonant method, see References 2 to 4).
As described in References 2 to 4, in a contactless power feeding technique using a magnetic resonance method, a device that receives power (hereinafter, referred to as power receiving device) and a device that feeds power (hereinafter, referred to as power transmitting device) each have a resonant coil. Further, in each of the power receiving device and the power transmitting device, an electromagnetic induction coil is provided. Feeding of power from a power source to the resonant coil in the power transmitting device and feeding of power from the resonant coil to a load in the power receiving device are conducted by the electromagnetic induction coils.
In the resonant coil of the power transmitting device and the resonant coil of the power receiving device, resonance frequencies (LC resonance) are adjusted so that a magnetic resonance phenomenon occurs at a particular frequency.
When the resonant coil of the power transmitting device and the resonant coil of the power receiving device face each other, they makes a magnetic resonance phenomenon, and thereby efficient power transfer is realized even when the distance between the resonant coils is large (see Reference 6).
In recent years, identification technologies in which ID (identification number) is assigned to an individual object to clarify the history of information thereof, which is utilized for production, management, or the like have attracted attention. In particular, RFID (radio frequency identification) technology for transmitting and receiving data by wireless communication has been widely used (see Reference 5). A wireless communication system using the RFID technology is a communication system including a wireless communication device (interrogator) and a data carrier (transponder) and communicating data wirelessly therebetween. The wireless communication device refers to a device capable of wirelessly transmitting and receiving signals, such as a reader/writer, a cellular phone, or a personal computer. In this specification, the wireless communication device is typically referred to as a reader/writer. In addition, the data carrier is generally called an RFID, an RF tag, an ID tag, an IC tag, an IC chip, a wireless tag, an electronic tag, or the like. In this specification, the data carrier is typically referred to as an RFID.
Examples of RFIDs include passive RFIDs which do not have power sources and active RFIDs which incorporate power sources. The passive RFID has a function of converting a wireless signal (carrier wave or amplitude-modulated wave generated by superimposing modulation wave on carrier wave) transmitted from a reader/writer into a DC voltage by a rectifier circuit which is provided in the RFID, and circuits in the RFID operate using the DC voltage.